The present invention relates to a space diversity receiving system in which carrier waves received by a plurality of antennas are combined under in-phase state at an intermediate frequency stage, and more particularly to a space diversity receiving system wherein all phase detection is performed outside of the main intermediate frequency channels, thereby eliminating low frequency phase modulation components from the combined intermediate frequency output.
Generally, to reduce the effects of fading and thereby to assure highly reliable communication, a space diversity receiving system is employed in a microwave communication system, in which a single transmitted electromagnetic wave is received by two or more antennas to allow the received carrier waves to be combined under in-phase state. Phase control means is needed to achieve the in-phase relationship among the received carrier waves. This phase control means is provided generally at a high frequency stage or at an intermediate frequency stage prior to the demodulation into a baseband signal. The phase control at high frequency stage has been in broader use, because of the simplicity in its construction. In the high frequency phase-controlled combination system, frequency-modulated or phase-modulated carrier waves received, for instance, by a first antenna and a second antenna are respectively band-filtered and, if necessary, amplified at a high frequency stage. After one of them has been phase-modulated by a low frequency signal and the other has been passed through a variable phase-shifter, they are both applied to a combiner to be combined. The output of the combiner is converted into an intermediate frequency signal by a frequency converter with a local oscillation applied to it. After amplification at an intermediate frequency pre-amplifier, the signal is applied to a main intermediate frequency amplifier having an automatic gain control circuit. From the output of the main intermediate frequency amplifier is obtained an output having a constant amplitude. Since the carrier wave received by one antenna is phase-modulated at the phase-modulator with the output of the low frequency oscillator, its phase difference from the phase of the carrier wave received by the other antenna results in a low frequency amplitude modulation component corresponding thereto appearing at the output of the main intermediate frequency amplifier. By controlling the amount of phase-shift of the above-mentioned variable phase-shifter in response to this low frequency component, the two received carrier waves are brought to an in-phase state at the input of the combiner.
However, in such a phase-controlled combining system, the fact that one of the received carrier waves in phase-modulated with a low frequency signal causes a low frequency phase-modulated component to be contained in the output of the combiner. Consequently, the frequency bandwidth allowed for the base band signal is limited, with the lower frequency region rendered useless. The recent trend toward the utilization of broadband transmission systems handling not only the multiplexed telephone signals, television signals and data signals, but also the command and/or control signals in the relay system has increased the need for the lower frequency region. However, the mere application of the above-described phase-controlled combination system to such transmission systems would result not only in the degradation of a signal-to-noise ratio of the baseband signal but also in the deterioration in the quality of other additional signals, because of the cross-modulation distortion caused by the low frequency phase modulation component.